I cannot really explain why the trigger did that to you. I did not personally experience this issue neither did other members complain.

However I do agree that the next logical step is to disable the trigger.

Sign into your Linux account as Oracle

Source the environment.
cd PROD
cd ap*
cd ap*
cd ap*
. APPSPROD_linux

Now go to sqlplus
sqlplus apps/apps

Find out the name of the trigger on FND_USER table.
select user, trigger_name from all_triggers where table_name = 'FND_USER';

Disable the trigger
alter trigger <USER>.<TRIGGER NAME> disable;

I am writing all these commands on the fly so you might have to tweak them a bit if there are errors.